Method of separating alkali-earth metal phosphates from primary alkali phosphate solutions



Patented Jan. 21, .1930

UNITED STATES PATENT OFFICE FRITZ DRAISIBACH, LUDWIGSHAFEN-ON-THE-RHINE, GERMANY, ASSIGNOR TO THE GERMANY METHOD ,OF SEPARATING ALKALI-EARTH METAL .PHOSPHATES FROM PRIMAR Y ALKALI PHOSPHATE SOLUTIONS Nolirawing Application fiied'February 29, 1928, Serial No. 258,132, and in Germany June 21, 1926.

tains alkalimetal phosphates.

phosphate solutions contain more or less In the said process of producing alkali metal phosphates by treating crude phosphates with sulfuric acid and neutralizing the solution by means ofsodium carbonate or 'the like, it is known that the resulting monophosphates of the. alkaline-earths and that the presence of the latter in the solution is very objectionable, if it is desired to produce 20 pure mono -alkali-metal phosphates in an economical way.

As a result of my research I found that even in case of highly concentrated solutions of mono-alkali-metal phosphates" such as solutions of mono-sodium-phosphate, monopotassium phosphate and mono-ammonium phosphate, it ispossible to convert any monoalkaline-earth phosphates contained in the said solutions into dialk ali-earth phosphates by adding di-alkali phosphates in the calculated quantity or preferably in a quantity slightly in excess thereof, and heating the same in an autoclave at high pressure as e. g. 5 to 15 atmospheres up to temperatures pressure requires a considerable increase in temperature. As the ionization of water alters very inuch with the temperature (at 156 C. it is 200 times as great as at 25 (3.), then the mono alkaline earth phosphate can be hydrolytically split by itinto di-alkaline earth phosphate and free phosphoric acid. The free phosphoric acid thus produced is bound by the di-alkali metal phosphate present. Thus for example the process for magnesium phosphate in a mono" potassium phosphate solutionin which any alkaline earth metals can be substituted for magnesi- 0 um and any of the alkali metals for potasbetween 150 and 200 C. The increased sii'nn, can be explained by the following equations MgH4P:Cr+MgHPO4+H;PO4 (hydrolytie s piitting) HaPOrkK HPOr-QKHPO; (hydrolytic splitting) From these equations it can be seen that in order to separate 40 parts of MgO as MgHPO 71 parts of P 0 in the form of alkali phosphate must beadded. Thus for example the process for magnesiumphosphate in a mono potassium phosphate 'SOl IllZlOII to which any alkaline earth metals maybe substituted for magnesium and any of the alkali metals for potassium, can be explained by the following equations:

MgILPzOa- MgHPW-l-H POi (hydrolytic splitting) HgPOrl-KgHPOrfiKHPO; (hydrolytic splitting) tity of di-sodium phosphate to a solution of mono-sodium phosphate containing 470 grams of P 0 9 grams of C210 and 32 grams of MgO in-the'litre, and I then heated the same for six'hours at a pressure of atmospheres up to 170 0., with the result that all of the mono'calcium phosphate and about 90 percent of the mono magnesium phosphate were converted into di-phosphates which are insoluble and show excellent filtering qualities so that they are easily. to be separated from the alkali metal phosphate in solution.

Simultaneously with the mono-earth-alkali phosphates any small quantities'of the dialkaline earth phosphates which may be present are also removed.

By this way it is possible to produce solutions of mono alkali metal phosphate entirely free from calcium phosphates and which contain only 0.4 to' 0.7 parts of MgO to 100 parts of P 0 sequently many intermediate crystallizations must be carried out .to obtain a pure clear soluble crystallized mono-alkali; phosphate are so completely removed by the process described in the present application, that the mono-phosphate solution is absolutely free fr'om" calcium oxide and per 100 parts of P only contains 0.2 to 0A parts of MgO.

A-particular economic. invaluable adYan tage ofthe new process is that it permits even strongly concentrated solutions likewise to be com letely purified from calcium oxide and Mg Hitherto these solutions had to be con siderably diluted in order to be able to carry out a urifi'cation from CaO and MgO as the solubi ity of theearth alkali phosphate inadvantages and novel features tion will be readily creases with increasingconcentration of the mono-alkali phosphate solution.

From the foregoing, it is believed that of my invenunderstood f" 'What- I claim is:

1. The method of separating phosphates of the alkaline-earths from solutions of primary alkaline phosphates containing such alkalineearth phosphates, consisting in adding dialkali phosphate to the said solutions and heating the mixture at above atmospheric pressure.

' 2. The method of separating phosphates of v the alkaline-earths from solutions of primary alkaline phosphates containing such alkalineearth phosphates, consisting in adding dialkali phosphate to the said solutions and heating the mixture at a pressure of between 5 and 15 atmospheres.

In testimony whereof I afiix my signature.

FRITZ DRAISBACH.

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